From what I understand from the movie, Captain Willard's journey up the Nung River to the heart of darkness is a metaphor for a human's descent into madness and loss of morality and the 3 stops (Kilgore, Playmates and The Bridge) in Willard's journey represent various stages of the dying humanity (embracing violence and losing remorse, losing innocence and empathy, losing purpose) with the final destination being the literal Heart of Darkness. How does the French Rubber Plantation scene fit in all of these?
I have only seen the Redux version once, but what I recall is that the plantation segment might be seen as purgatorial. There is a burial, the French seem stuck in time, and the scene with the widow ends with her behind a shroud as a ghost.
On top of that, I have seen references (e.g. Ebert review) asserting that Coppola described the French as ghosts. Cannot find a direct quote though.
The widow repeatedly tells Willard that he is both an animal and a god. Not sure how to frame that into the context you have provided.
Since the theme of Apocalypse Now (as in Conrad's Heart of Darkness) is corruption as a process, that process being primarily that of colonialism, with the of course originally Victorian preoccupation being the corrupting effect upon the colonizer - the setting of a plantation - a place for managing and living on the expropriation, slavery, misery, ecological and economic debasement - is certainly apropos. On the way to chaos, but still (at one time) in good brutal working order.